What forms of Buddhism don't require prostrations, recitations, prayers, yoga stretches, candles, incense, statues, malas and all other forms of external, sensuous practice?
Well, maybe Theravada or zen are the most spare in terms of "techniques." Check out dhamma.org for a really good vipassana retreat.
But even the barest form of sitting meditation is still an "external, sensuous practice" - because it requires you to do something with your body and engage the senses in a particular way (usually, by introverting them). Mahayana teaches that you can attain enlightenment in 16 lifetimes just by practicing kindness, without even doing any meditation (according to Words of My Perfect Teacher
)- but this still requires external activity.
Maybe Dzogchen would actually be the closest to what you're asking about - the essential "practice" in Dzogchen is Guru Yoga - usually this is taught with some kind of visualization, but even that isn't necessary - if you can rest in the natural state with ease. Visualization would be just a tool. If you can walk around 24/7 in rigpa, that would be the ideal - nothing else needed. But apparently all reputable teachers of all schools seem to have found that some sort of "practice" is necessary to tame our unruly minds. Especially in these degenerate times, which is why Tantra is supposed to be so effective these days. . .